A number of means and methods for detecting the velocity of an object are known in the prior art. One such method projects broad spectrum light reflected from a rough illuminated moving surface onto a mask imprinted with regularly spaced non-transparent lines. The mask is viewed by a detecting means. Each quantum of the returned signal has its own phase relative to an arbitrary reference signal, and the projection lens effectively sums these quanta into a single signal, the overall phase and frequency being related to the velocity of the surface. Whilst this technique delivers a velocity signal, accuracy relies on smoothing the signal, as frequency variations occur which relate to the surface roughness and not just the surface velocity. Also, direction sensing is not possible with this technique.
A further prior art technique known as Laser Doppler Velocimetry (“LDV”), uses the coherent nature of laser light to focus two crossing laser beams with identical polarisation at a single reference point, thereby creating linear and regularly spaced interference fringes within a defined measurement volume. An object passing through the measurement volume will reflect incident light from the fringes back to a detector via a lens system. The signal frequency will relate to the fringe spacing and the velocity of the object. For precision laser beam geometry, fringe spacing is highly regular, allowing accurate velocity measurements to be made. However, it is not possible to determine the direction of transit, as the frequency signal is identical for both left-to-right or right-to-left transitions. Nor is it possible to determine if the object path is exactly orthogonal to the fringes, a condition which produces the highest frequency. For a transit at an angle to the fringes, the frequency will be lower than the theoretical maximum by the cosine of the transit angle.
Complex optoelectronic means, for example Bragg cells, have been employed in LDV systems to modulate the frequency of one of the two laser beams, thereby changing the fringe spacing at the modulation frequency. It is possible to determine L-R or R-L direction by mathematical interpretation of the returned signal. This and similar methods are complex, expensive and not very compact, often requiring high voltages and producing a noisy returned signal, thereby decreasing measurement accuracy.
To measure velocity in X and Y axis, two orthogonal fringe patterns must be created and superimposed in the same measurement volume. In order to distinguish each axis, different laser wavelengths are used for each axis, and the returned signal must be split and filtered at each laser wavelength and then detected. To measure velocity in the Z axis, either an interferometer system must be created, or a second, single plane LDV unit with a different laser wavelength is used, mounted to focus its measurement volume exactly on the two plane volume, but at a defined angle, thereby allowing a Z axis velocity component to be measured. For a small measurement volume, perhaps only 100 μm in diameter, and a long working distance, perhaps over 1 metre, this requires extreme precision.
To produce a single assembly which contains all of the laser beam generation at three different wavelengths with a precision single measurement volume, each having (Bragg) modulation systems in one of the laser beams, together with modulation electronics, high voltage power supplies, three optical receivers at different wavelengths and three detection systems is a difficult and expensive engineering task.